1. Field of the Invention
The present invention generally relates to optical axis deflecting methods, optical axis deflecting elements, optical axis deflecting units, methods of driving optical axis deflecting element and image display apparatuses, and more particularly to an optical axis deflecting method and an optical axis deflecting element for changing a tilt direction of an optical axis of a uniaxial material in response to an electrical signal, an optical axis deflecting unit for deflecting an optical path of light in response to an electrical signal using such an optical axis deflecting element, a method of driving such an optical axis deflecting element, and an image display apparatus which uses such an optical axis deflecting element or unit. The present invention also relates to an optical path deflecting method, an optical path deflecting element and an optical path deflecting unit using such an optical axis deflecting element.
2. Description of the Related Art
In this specification, an “optical path deflecting element” refers to an optical element capable of deflecting an optical path of light in response to an external electrical signal, by shifting outgoing light in parallel with respect to incoming light or, by rotating the incoming light by a certain angle or, by combining the parallel shift and the rotation of the incoming light, so as to switch the optical path. An amount or magnitude of the parallel shift of the optical path deflection is referred to as a “shift quantity”. An amount of the rotation of the optical path deflection is referred to as a “rotation angle”. An “optical path deflecting unit” refers to a device which includes the “optical path deflecting element” and deflects the optical path of light.
An image display apparatus may include at least an image display element having a plurality of light controllable pixels arranged two-dimensionally, a light source for illuminating the image display element, an optical member for monitoring an image pattern displayed by the image display element, and an optical path deflecting means for deflecting light in the optical path between the image display element and the optical member for each of a plurality of sub fields into which an image field is divided time-divisionally. This image display apparatus can display an image by increasing the apparent number of pixels of the image display element, by displaying an image pattern having a display position that is shifted depending on the deflection state of the optical path for each of the sub fields caused by the optical path deflecting means. A “pixel shift element” refers to such an optical path deflecting means. Basically, the “optical path deflecting element” and the “optical path deflecting unit” can be applied as the “optical path deflecting means (pixel shift element)”.
Conventionally, various optical path deflecting elements and pixel shift elements using liquid crystal materials, and image display apparatuses using such elements, have been proposed. Examples of the proposed elements and apparatuses may be found in Japanese Laid-Open Patent Applications No. 6-18940, No. 9-133904, No. 5-313116, No. 6-324320 and No. 10-133135, and Japanese Patent No. 2939826. But according to the conventional optical path deflecting elements and pixel shift elements, the following problems (1) through (3) were encountered.
(1) Because of the complex structure of the elements, the elements are expensive and the size of the elements are large. In addition, the optical loss, optical noise such as ghost, resolution and the like deteriorate.
(2) In the case of a structure having a movable part, the positioning accuracy and the durability deteriorate, and problems such as vibration and mechanical noise increase.
(3) In the case of a structure using a nematic liquid crystal, it is difficult to obtain a high response speed.
A Japanese Laid-Open Patent application No. 2002-328402 proposes optical path deflecting elements and pixel shift elements using liquid crystal materials, and image display apparatuses using such elements, that are designed to suppress particularly the problems (1) described above. FIG. 1 is a cross sectional view showing an optical path deflecting element proposed in the Japanese Laid-Open Patent application No. 2002-328402.
An optical path deflecting element 1 shown in FIG. 1 includes a pair of transparent substrates 2 and 3. An orientation layer 4 is provided on at least one of the substrates 2 and 3, and a liquid crystal layer 5 having a chiral smectic C-phase forming a homeotropic orientation is provided between the substrates 2 and 3. Electrodes 6a and 6b forming an electrode pair 6 are provided at ends of the substrates 2 and 3, so as to apply an electric field on the liquid crystal layer 5 when a voltage from a power supply 7 is supplied across the electrode pair 6. Since the optical path deflecting element 1 uses the liquid crystal layer 5 having the chiral smectic C-phase, it is possible to suppress the problems (1) described above. In addition, it is also possible to suppress the problems (2) and (3) described above.
In the case of the optical path deflecting element 1 shown in FIG. 1, if the electric field is applied in a direction perpendicular to a spiral axis of the chiral smectic C-phase, that is, in a direction parallel to the liquid crystal layer 5, it may be regarded that liquid crystal molecules undergo a rotary motion within a cone-shaped virtual surface within the liquid crystal layer 5. In this state, a proportion of the liquid crystal molecules that are oriented in the same direction changes depending on characteristics such as the spiral pitch and spontaneous polarization of the liquid crystal layer 5, and a tilt direction of the optical axis of the liquid crystal layer 5 corresponding to an average orientation direction of the liquid crystal molecules changes.
As described in the Japanese Laid-Open Patent Application No. 2002-328402, when a conoscope image of the liquid crystal layer having the chiral smectic C-phase and not applied with an electric field is observed in a direction normal to the liquid crystal layer using a polarizing microscope, a cross image is located at a central portion, and a uniaxial optical axis of the liquid crystal layer can be confirmed. FIG. 2 is a diagram showing a model of the liquid crystal molecule arrangement of the chiral smectic C-phase, for showing a change in the spiral structure due to the applied electric field. In the model of the liquid crystal molecule arrangement, molecular layers having a tilt angle θ form the spiral structure by being mutually shifted and overlapping with each other.
When the applied electric field E is E=0, the spiral structure has a symmetric structure to the right and left as shown in FIG. 2(a), and the liquid crystal director direction is spatially averaged. The average optical axis of the liquid crystal layer is aligned in the direction normal to the liquid crystal layer. Hence, the liquid crystal layer is optically isotropic with respect to incoming light parallel to the average optical axis.
When the applied field E is parallel to the liquid crystal layer and is relatively small such that a relationship 0<E<Es is satisfied, where Es is a threshold value of the electric field E, an angular moment acts on the liquid crystal molecules due to the action of a spontaneous polarization Ps on the electric field E. As a result, the spiral structure becomes distorted and non-symmetrical as shown in FIG. 2(b), and the average optical axis is tilted in a direction. In this state, the distortion increases as the electric field intensity increase, to thereby increase the tilt angle of the average optical axis. The increase in the tilt angle of the average optical axis may be observed from a positional shift of the cross image in the conoscope image.
When the electric field intensity is further increased and the applied electric field E becomes greater than or equal to the threshold value Es (that is, E≧Es), the spiral structure disappears, and optically, the liquid crystal layer becomes approximately uniaxial as shown in FIG. 2(c). In this state, the tilt angle of the optical axis becomes equal to the tilt angle θ of the liquid crystal director. This tilt angle θ does not changed even if the applied electric field E is further increased, and the tilt angle of the optical axis becomes fixed.
Therefore, when a sufficiently large electric field E is applied on the liquid crystal layer, the orientation directions of the liquid crystal molecules within the liquid crystal layer become aligned, and the spiral structure disappears. In addition, when the direction of the applied electric field E is reversed, the tilt direction of the optical axis of the liquid crystal layer is also reversed. Thus, the liquid crystal layer can function as an optical axis deflecting element or a dynamic double refraction plate, and may be applied to optical path deflecting elements, optical deflecting units and the like.
When the optical path deflecting element or the optical axis deflecting element is applied to the image display apparatus and the like, it is necessary to design the width of the effective region of the liquid crystal layer for transmitting the light to a large value. But when an attempt is made to apply an electric field which is parallel to the liquid crystal layer (that is, parallel to the substrate surface) with a sufficiently large intensity for driving the liquid crystal layer, an extremely large voltage on the order of several kV must be supplied across the width of the effective region if the width of the effective region is greater than or equal to several tens of mm and large.
However, when the extremely large voltage on the order of several kV is supplied across the width of the effective region of the optical path deflecting element, discharge and noise may be generated within the image display apparatus. In addition, the size of the required power supply increases, and the power consumption increases.